The present invention relates to the preparation of thin, precursor ceramic tapes, and to the conversion of said tapes into thin ceramic sheets. The thin ceramic sheets of the present invention are useful in the preparation of multilayer capacitors and other electronic devices, such as multilayer substrates. A multilayer capacitor is a stack of several ceramic capacitor sheets connected in parallel to give a capacitor having high total capacitance. They can be small in size. For example, a typical multilayer capacitor size is 2 mm.times.4 mm.times.1 mm thick. Thus, these devices are ideal for microcircuitry. The manufacturers of multilayer capacitors desire to reduce the thickness of each individual ceramic sheet to approximately half of the 20 micron thickness which typically is used today. By doing so, the manufacturers could halve the number of ceramic sheets required for each capacitor, since capacitance is inversely proportional to the thickness of the ceramic sheets employed.
An additional advantage of reducing the number of ceramic sheets in each multilayer capacitor is that the number of layers of electrode is proportionally reduced. This would result in a large savings in material costs, as the electrodes consist of expensive noble metals.
In current commercial practice, multilayer capacitor manufacturing involves first dispersing ceramic dielectric powders consisting mainly of barium titanate and additives such as strontium titanate, lead titanate, calcium zirconate, lead oxide, borates, and silicates, into an organic solvent, such as methyl ethyl ketone/ethanol, by ball-milling the dispersed powder overnight. The resulting suspension of well-dispersed powders is then mixed with polymeric binders, plasticizers, and surfactants to form a slip. The slip is tape-cast onto a nonporous substrate, and is dried in an oven to produce a flexible sheet called a green tape. The green tape is screen-printed with a noble metal electrode. Forty to fifty layers of the printed tape are then laminated. The resulting laminate is diced into chips, and the organic binder is burned off. The resulting inorganic article is sintered at high temperatures typically ranging from about 1000.degree. C. to 1300.degree. C. to give a mechanically and electrically acceptable device.
The most common problem that multilayer capacitor manufacturers face is quality and consistency of ceramic powders. It is well-known that, in addition to high purity required for each ceramic compound, ceramic particles must be small in size, uniformly sized, and equiaxially shaped. In reality, ceramic powders typically are agglomerated when received, and even with lengthy ball-milling, slips still contain some large agglomerates. Accordingly, it would be desirable to have a process for the preparation of ceramic slips and free-standing precursor tapes which would eliminate ball-milling or other milling steps, and which would eliminate the need for ceramic powders and their accompanying quality problems.